Nationals Senator Bridget McKenzie from the Parliamentary friends of shooting group, at the Canberra International Clay Target Club on Friday 23 October 2015. Photo: Alex Ellinghausen

Gun-loving Nationals senator Bridget McKenzie will be catapulted from the backbench straight into federal cabinet after being named as Barnaby Joyce's new deputy.

But her elevation could lead to a demotion for one of her Nationals colleagues - Matt Canavan, Darren Chester or Nigel Scullion - in Prime Minister Malcolm Turnbull's coming reshuffle.

With former deputy leader Fiona Nash set to be replaced in the Senate by a Liberal, the junior Coalition partner is now strictly only entitled to four seats in cabinet, rather than five.

With Mr Joyce and Senator McKenzie guaranteed a spot in the inner circle because of their leadership roles, that leaves space for only two more - unless Mr Turnbull chooses to make an exception and keep all five. Mr Turnbull is expected to reshuffle his ministry in the coming weeks, after the Bennelong byelection.

Senator McKenzie beat out Resources Minister Matt Canavan - a close ally of Mr Joyce, who used to work as his chief of staff - for the coveted role. It's understood a number of other MPs also put their hands up for the job, including Michael McCormack and Keith Pitt.

The vote went ahead after the High Court made it clear on Wednesday there was no path back into the Senate for Ms Nash, whose seat will instead go to Liberal Jim Molan. It also follows on from the New England byelection that returned Mr Joyce with a thumping majority.

Senator McKenzie, 47, is an avid shooter and hunter who dislikes the "snobbery and elitism" of city dwellers who turn up their noses at firearms. But she drew some embarrassing attention last year after it emerged she was actually living in the leafy inner Melbourne suburb of Elwood, rather than in the regions.

But she says that's no reason to doubt her regional credentials.

"It was a little one bedroom I was living in while I did it up - and once I did it up I went back to Ballarat," she told Fairfax Media. "I identify as a country Victorian. In my 48 years I have probably lived in country Victoria for 40 of them. Every ounce of my being, my world view, my value system is regional."

She's not naming any particular portfolios she's after, only that she wants something that lets her "get out on the ground in the regions and deliver tangible benefits".

Senator McKenzie's promotion came as Federal Parliament was poised to finally legalised same-sex marriage - a reform she opposed despite having a gay younger brother, Alastair.

"Growing up in the country can be tough and isolating, growing up gay in the country in the 1990s was horrific," he wrote in a letter to the Bendigo Advertiser. "Given her own story and connection, I had hoped to see a more courageous and compassionate response."

But Senator McKenzie does not want to talk about her family: "I voted on the same-sex marriage legislation according to my own conscience. I don't have any more comment to make."

Mr Joyce welcomed Senator McKenzie's elevation, saying she was chosen based on talent rather than gender but it was "pretty handy" she represented "half of humanity".

The Nationals leader once described Senator McKenzie as a "flash bit of kit" during a late night Senate debate, prompting accusations he was drunk in the chamber - and an eventual apology.